Method of making grain oriented electrical steel sheet

ABSTRACT

A method for producing oriented electrical steel sheets containing less than 1.8 percent by weight of silicon is disclosed wherein from 0.001 to 0.100 percent by weight of selenium is added to molten steel, and wherein the hot rolled sheet is normalized at a temperature above A3 transformation point of the steel and is cold rolled to the final gauge with a single stage of cold rolling process.

Umted States Patent [191 [111 3,802,936 Goto et al. [451 Apr. 9, 1974 METHOD OF MAKING GRAIN ORIENTED [56] References Cited ELECTRICAL STEEL SHEET UNITED STATES PATENTS [75] Inventors: Isamo Goto, Kobe; lsao Matoba, 3,157,538 11/1964 lmai et a1 148/112 Ashiya; Shigeo Kinoshita, g t;

aauraeta... 25$???'gmgfi'figzgi 3,333.993 8/1967 Kohler Tomomichi Gem k iTumihiko 3.556.873 l/l97l Malagari l48/3l.55 Takeuchi Kobe of Japan Primary Emminer-Richard J. Herbst [73] Assignee: Kawasaki Steel Corporation, Assistant E.\'aminerD. C. Reiley, lll

Fukiai-ku, Kobe City, Japan Attorney, Agent, or FirmRobert E. Burns; Emman- 22 Filed: Mar. 7, 1972 211 Ap l. No.: 232,437 571 ABSTRACT Related US Application Data A method for producing oriented electrical steel sheets containing less than l.8 percent by weight of [63] fgg g ggg glg of silicon is disclosed wherein from 0.001 to 0.lO0 percent by weight of selenium is added to molten steel, UIS. I I I I I U 7 and wherein the hot rolled Sheet is normalized at a 51 Int. Cl. i101: 1/04 temperature abve "ansfirmatim of Search U 1 steel and iS cold rolled to the final gauge with a single (A) Not containing stage of cold rolling process.

3 Claims, 7 Drawing Figures (8) Containing 0.0|3% S0 ATENTEDAPR 9 m4 3. 802,936

SHEET 1 or 6 Fig. l (KG) I90} 0 0.625 0.650 0.675 OJbO 0%) Se content Fig. 2

(B)Comoining (A) g? confommg 003% S8 ATENTED PR 19M 3.802.936

' sum 3 0F 6 Fig. 4

.2 63 C U! i -No normclhzonon --Dec0rbyrizc|fi0n unneahng I;

800 900 I000 n00 (c) Normalization temperature (Heating timez5min.)

mmnznm 9:914 3302.936

SHEET 3 OF 6 Fly. 5 4 2 Thickness of hot Yne/cm rolled sheet (mm) -X-X- L5 2.0 g

-9 E IO- Cold rolling reduction ATENIEOAPR 91974 3802' 936 SHEET 5 UF 6 I Fig. 6

(/00) Pole figure METHOD OF MAKING GRAIN ORIENTED ELECTRICAL STEEL SHEET This is a continuation-in-part of application Ser. No. 815,788, filed Apr. 14, 1969, now abandoned.

This invention relates to a method for producing oriented electrical steel sheet of low silicon steel or mild steel having a-y transformation. It is an object of the invention to develop the preferred orientation known as cube-on-edge grain orientation in the steel sheets and to provide a steel sheet having a high permeability in the rolling direction and more particularly to provide a steel sheet having a high permeability at high induction. 7

It has heretofore been proposed to produce oriented silicon steel industrially from a raw material mainly consisting of high silicon steel containing about 3 percent silicon and having no a'y transformation. Such proposed method makes use of suitable cold rolling and final high temperature annealing processes so as to obtain secondary recrystallization texture having (110) [001] orientation and excellent magnetic properties in the rolling direction.

In order to improve the magnetic properties of oriented silicon steel, the role of some kinds of impurity elements added to the steel has been highly investigated and a proper amount of the additional elements has been found to be able to inhibit normal grain growth of primary recrystallization grains as mentioned in the US. Pat. No. 2,802,761,No. 2,867,558 and No. 3,157,538. These convenient additional elements are usually applied to high silicon steel containing about 3 percent silicon and make it possible to produce excellent grain oriented silicon steel sheets by two stages of cold rolling processes. But it is not always believed that these additional elements which act as an inhibitor for the normal grain growth of primary recrystallization grains are also effective in low silicon steel having a-y transformation.

In general, it has been well-known that, when a steel sheet is cooled from a temperature above the A transformation point of the steel to a room temperature, the crystal structure formed at an elevated temperature above A transformation point changes into a fine grain structure having a random orientation due to 04- transformation. The low silicon steel or mild steel generally passes through this transformation point during or after hot rolling process so that the crystal structure of the hot rolled sheet is entirely different from that of the high silicon steel having no a-y transformation. That is, the structure of the high silicon steel generally consists of a recrystallization region at the surface layer of the sheet and a fiber structure having intense (001) [l orientation at the central layer of the sheet, and the nuclei of the recrystallization grains having (1 10) [001] orientation are said to be formed mainly at the surface region during the cold rolling and subsequent annealing processes. On the other hand, as for the hot rolled structure of low silicon steel or mild steel the recrystallization grains are uniformly distributed as a whole, although weak (001) [110] orientation is still partly remained at the central region, and then the nucleus formation of (110) [001] recrystallization grain is not always limited only to the surface layer. The growth mechanism of the (1 10) [001] grains becomes inevitably different between in both kinds of steel, and consequently the different cold rolling and annealing processes are required for the development of the (1 10) [001] preferred orientation in both kinds of steel.

The inventors have carried out a number of experi ments in order to develop the preferred orientation in low silicon steel or mild steel having a-y transformation and found that electrical seel sheets having an excellent magnetic property in the rolling direction can be produced by a combination of the processes of adding from 0.001 to 0.100 percent selenium to molten steel, casting said selenium added molten steel, blooming and hot rolling into a sheet having a thickness of from 1.5 mm to 3.0 mm, normalizing said hot rolled sheet at a temperature above the A transformation point of the steel to make the carbon content at least 0.010 percent, cold rolling said normalized sheet with a reduction of from 35 to percent in the thickness, and finally annealing said cold rolled sheet.

It has been well-known that selenium acts as an inhibitor for the normal grain growth of primary matrix grains. The inventors have found out that the selenium not only acts as an inhibitor, but also interacts with so]- ute carbon to increase the sites for the precipitation of carbide. The fine precipitates of carbide formed after the normalizing of the hot rolled sheet accelerate cross slip and tangling of dislocation during the cold rolling, and so the cell structure can be obtained at a fairly lower reduction in the steel containing selenium as compared to in the steelwithout selenium. It has been also found that the steel containing selenium has a smaller cell size and more nuclei of l 10) [001] recrystallization grains in the cold rolled structure than the steel without selenium. Thus, if the selenium is added to low silicon steel or mild steel and the solution treatment of carbon, that is, normalizing is performed before cold rolling process, it becomes possible to obtain the secondary recrystallization texture having [001] orientation after final annealing by means of a single stage of cold rolling process with a low reduction 1 at which it has been ordinary considered impossible to obtain such texture in the steel.

The normalizing performed at a temperature above A transformation point of the steel aims to increase the soluble carbon concentration in the hot rolled sheet, and at the same time to decrease the (001) [1 l0] orientation partly remaining at the central region of the sheet because the existence of the (001) [110] grains in the hot rolled sheet is apt to disturb the growth of (110) [001] grains during the annealing process after cold rolling.

The compositions of the raw material to be used and the conditions of each process treated according to the method of this invention will now be described in detail. Reference is made to the accompanying drawings, in which:

FIG. 1 shows graphically the effect of the selenium content in the mild steel on the magnetic flux density B after final annealing;

FIG. 2A and FIG. 2B show photographs taken with a magnification of l and illustrate crystal structures of the low silicon steel not containing selenium and containing 0.013 percent selenium, respectively, both of them have been obtained after final annealing;

FIG. 3 shows graphically a relation between the silicon content and the magnetic flux density B of the low silicon steel after final annealing;

FIG. 4 shows graphically a relation between the normalizing temperature before cold rolling and the magnetic torque of the steel sheet after final annealing;

FIG. 5 shows graphically the efrect of the cold rolling reduction applied to the hot rolled sheets having different thickness on the magnetic torque of the mild steel after final annealing;

FIG. 6 is the (100) pole figure, as determined by the X-ray diffraction method, of 20 crystal grains selected at random from the low silicon steel sheet produced by the method of this invention; and

FIG. 7 shows B-I-I curves illustrating the permeability of the steel sheet according to the invention at a high induction compared with those of the conventional steel sheets.

The steel ingot to be used as a starting material may be obtained by casting molten steel melted by means of an open-hearth furnace, electric furnace or converter. The molten steel should contain less than 1.8 percent silicon, more than 0.010 percent carbon, from 0.001 to 0.100 percent selenium and the remainder of iron and impurities. It is to be understood that all composition percentages set forth therein are weight percentages. In order to prevent hot brittleness of the steel, it is desired to be added from 0.04 to 0.20 percent Mn to the molten steel in dependence upon the selenium content.

The range of selenium contents as defined above has been determined by experiments in which from 0.001 to 0.098 percent selenium was added to the molten steel containing 0.025 percent carbon and 0.02 percent silicon. The relation between the selenium content and the magnetic flux density (B in the rolling direction) after final annealing was plotted in FIG. 1. It is apparent from FIG. 1 that the magnetic flux density is remarkably raised by the addition of more than 0.001 percent selenium. "The steel containing selenium has secondary recrystallization grains uniformly distributed in the steel-sheet, while the steel without selenium has no such secondary recrystallization grains at all, as shown in FIG. 2A and FIG. 2B, which are photographs taken with a magnification of l and illustrate crystal structures of both kindsof steel after final annealing. The main reason why the maximum amount of selenium content to be added is limited to 0.100 percent is that the addition of the excessive amount of selenium is not economical.

In accordance with the invention the silicon content is determined to 0 to 1.8 percent Si and more preferably to 0.5 to 1.3 percent Si for the purpose of giving a-y transformation to the oriented steel sheet and also of improving-the permeability at a high flux density. The preferable lower limit of 0.5 percent Si is so determined that the holding temperature of the final annealing to be described later can be made higher for a shorter time. i

The magnetic flux densities vary with the silicon contents of the steel sheets as shown in FIG. 3, which shows a relation between the magnetic flux density after final annealing and the silicon content of the steel sheets containing 0.030 percent carbon and 0.015 percent selenium before cold rolling. From this figure, it is recognized that the magnetic flux density of the 2.95 percent silicon steel having no 01- transformation is much lower than that of the steel having a-y transformation.

The molten steel having the above mentioned compositions is cast and hot rolled into a sheet having a thickness of from 1.5 mm to 3.0 mm by means of a conventional process inclusive of a blooming process, if necessary. The hot rolled sheet thus obtained is then normalized at a temperature above the A;, transformation point of the steel for about 5 minutes. If the normalizing temperature is below the A;

' transformation point and the carbon content of the hot rolled sheet is less than 0.010 percent, the finally annealed steel sheet could not have excellent magnetic properties. If the carbon content of the hot rolled sheet is too high to decarburize to below 0.005 percent by the decarburizing annealing before final annealing, it is preferable to decarburize the hot rolled sheet to the range of about'from 0.01 to 0.04 percent carbon by a suitable annealing process.

FIG. 4 shows the effect of the normalizing temperature on the magnetic torque of the mild steel sheet, the hot rolled sheet of which contains 0.024 percent carbon, 0.02' percent silicon and 0.016 percent selenium and has a thickness of 2.0 mm. For comparison, the chain lines in this figure indicate the torque value of the steel sheet which has not been normalized before cold rolling, and the dotted lines also indicate the torque value of the steel sheet which has been subjected to decarburization annealing at 700C for 15 hours such that the carbon content becomes 0.003 percent instead of normalizing of the hot rolled sheet. It is apparent from FIG. 4 that the hot rolled sheet should be normalized at a temperature above the A, transformation point of the steel so as to obtain excellent magnetic properties after final annealing.

The steel sheet which has been subjected to decarburization annealing such that the carbon content becomes 0.003 percent developed secondary recrystallization texture having (1 l 1) [112] orientation after the final annealing. The desired secondary recrystallization texture having (110) [001] orientation could not be obtained.

The hot rolled sheet thus normalized is subjected to a pickling treatment to remove the oxide film on the surface of the sheet and then cold rolled with a reduction of from 35 to 80 percent in the thickness. The range of the cold rolling reduction can be applied to the hot rolled sheet having a thickness of from 1.5 mm to 3.0 mm as shown in FIG. 5.

The cold rolled sheet is then decarburized in a wet hydrogen atmosphere tomake its carbon content less than 0.005 percent and coated with a separating agent such as magnesia so as to prevent the sheets from adhering each other during the final annealing. The final annealing should be carried out under such condition that secondary recrystallization grains having [001] orientation can well develop, and it is a matter of course that the annealing temperature should not ex- -ceed a region.

The upper limit of the annealing temperature not exceeding a region depends upon the amount of Si. If the amount of Si is decreased, the annealing temperature becomes lower. Consequently, the silicon steel containing 0.8 percent silicon can be box annealed at 900C for 24 hours in order to develop the desired secondary recrystallization grains having (110) [001] orientation, while the mild steel containing 0.1 percent silicon must be box annealed at 875C for 48 hours in order to obtain the same result, which is rather uneconomical.

The procedure of producing oriented electrical steel sheets by means of a single stage of cold rolling process with a reduction of more than 60 percent has been proposed in US. Pat. No. 2,287,466, but this method is substantially different from our invention because of the lack of the processes of the selenium addition and the normalizing prior of cold rolling.

There have been proposed several methods to produce oriented electrical steel sheets using low carbon steel, but most of them consist of two stages of cold rolling processes with an intermediate annealing, and the magnetic properties of the steel thus produced are much inferior to those of the steel sheets produced according to the method of this invention. For example, magnetic flux density B in the rolling direction of the steel sheet produced by the conventional method is about 18.0 Kg, while B of this invention is 20.3 KG, which shows remarkable superiority of this invention.

This invention has made it possible to apply a wide range of cold rolling reduction of from 35 to 80 percent and to produce an oriented electrical steel sheet having a thickness of from 0.30 mm to 1.7 mm by a single stage of cold rolling process. Thus, the invention provides an economical way for producing oriented electrical steel sheets having a thickness of more than 1.0 mm and can be applied satisfactorily to the electrical industrial field.

The invention will now be explained with reference to an example.

Example A low silicon stell containing 0.8 percent Si was prepared by a converter refining operation.

The silicon steel was formed into 7t of steel ingot added with Se. The chemical compositions of the steel ingot are shown in Table 1.

6 rate into a strip having a thickness of 1.0mm. The strip was treated to remove the rolling oil and then subjected to a continuous annealing in a 35%H +65%N gas hav- TABLE 1 35 Chemical compositions of steel ingot C 0. 025 Si 0. 78 Mn 0. 05 P 0. 006 S O. 004 So 0. 015 Al 0. 002 0. 0046 N 0. 0049 The steel ingot was heated in a soaking pit and was bloomed into a slab having a thickness of 150 mm. The slab was heated again and then hot rolled into a strip having a thickness of 2.0 mm. This strip was normalized in air at 1,000C for 5 minutes. After a pickling treatment for removing the surface oxide layer the normalized strip was cold rolled under a percent reduction ing a dew point of 50C at 820C for 15 minutes to make the carbon content 0.003 percent. The strip thus treated was coated with magnesia as a separating agent and was finally box annealed in dry hydrogen at 900C for 24 hours. Twenty grains were selected at random in order to determine the structure of the crystal grains of the strip subjected to the box annealing and the orientation of these twenty grains is shown by (1 10) pole figure in FIG. 6 which illustrates that all of these grains orient themselves near to the (1 10) [001] or Goss orientation.

As seen from B-T-l curves shown in FIG. 7 experimental tests have yielded the surprising result that the ,u. value (u=B/H) at 20 KG of the oriented steel containing 0.8 percent Si according to the invention is 1,180 which is far superior to 150 of the conventional,

1. A method of producing grain oriented electrical steel sheet of mild steel or low silicon steel containing less than 1.8 percent by weight of silicon and having a'y transformation, comprising the steps of providing a molten steel containing less than 1.8 percent by weight of silicon and having a-y transformation, adding from 0.001 to 0.100 percent by weight of selenium to the molten steel, casting the selenium added molten steel, hot-rolling the cast steel into a sheet having a thickness from 1.5 mm to 3.0 mm, normalizing the hot rolled sheet at a temperature above the A transformation point of the steel to make the carbon content 0.010 to 0.04 percent, cold rolling the normalized sheet in a single stage of cold rolling, decarburizing the cold rolled sheet in a wet hydrogen atmosphere to make its carbon content less than 0.005 percent and finally annealing the decarburized cold rolled sheet, whereby grain oriented electrical steelsheet having excellent magnetic properties in the rolling direction is produced.

2. The method as claimed in claim 1, in which the low silicon steel contains 0.5 to 1.3 percent by weight of silwon.

3. The method as claimed in claim 1, in which the hot rolled sheet after normalizing is reduced to final gauge in a single stage of cold rolling producing a reduction of from 35 to percent. 

2. The method as claimed in claim 1, in which the low silicon steel contains 0.5 to 1.3 percent by weight of silicon.
 3. The method as claimed in claim 1, in which the hot rolled sheet after normalizing is reduced to final gauge in a single stage of cold rolling producing a reduction of from 35 to 80 percent. 